Infrared burner

ABSTRACT

An infrared burner with an extremely low pressure drop is described. The burner comprises a corrosion resistant mesh screen having a thick porous coating of ceramic fibers deposited thereon. In addition to a low pressure drop the burner has structural integrity and ability to generate radiant energy at a high rate. The burner is produced by admixing ceramic fibers with a polymeric material which burns off upon heating.

DESCRIPTION

1. Technical Field

The field of art to which this invention pertains is infrared burners,and specifically composite infrared burners.

2. Background Art

Radiant energy burners made up of a supporting screen coated with amixture of ceramic fibers are known. In operation the fibers areassembled such that they are permeable to gaseous fuel and the fuel iscombusted on the outer surface of the element to primarily emit radiantenergy. A variety of designs and methods for making such burners isdescribed in the art, note U.S. Pat. Nos. 3,179,156; 3,275,497;4,519,770; 4,599,066; and 4,721,456. However, there is a constant searchin this art for more efficient burners as energy costs rise.

DISCLOSURE OF INVENTION

An infrared burner is described comprising a corrosion resistant meshscreen having deposited thereon a thick porous coating of ceramicfibers. The fibers are selected in size and distribution so that theresultant burner has structural integrity, and the ability to releaseradiant energy at the rate of 80,000 to 100,000 BTUs per sq.ft. perhour. The porosity of the fiber laying is such as to produce adifference in pressure drop during operation of less than 0.4 inch ofwater and a pressure drop cold versus hot of less or equal to 0.20 inchof water.

Another aspect of the invention is a method of making such burnerscomprising admixing colloidal alumina, ceramic fibers, and adecomposable polymeric material in a carrier. The mixture is keptuniformly suspended and a mesh screen substrate is immersed into themixture. A vacuum is pulled through the screen for a period of timesufficient to deposit a coating up to 0.5 inch thick. Followingdeposition, the coating is heated to decompose the polymeric material onthe surface of the coating and produce a porous coating on the screen.The resultant article has the properties described above, in addition tobeing smooth and structurally stable.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an infrared burner according to the present invention.

FIG. 2 shows a schematic of the process used to produce the articleaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The mesh screen can be any material which is corrosion resistant underthe gas and heat environment prevalent with an infrared burner of thistype. Commercially available stainless steel is most preferred. Whileany mesh size opening sufficient to hold the ceramic fibers whileallowing adequate porosity to accomplish the purposes of the inventioncan be used, mesh sizes less than 0.25 inch on a side are preferred,0.125 inch on a side being most preferred.

The ceramic fibers used are preferably commercially available hightemperature stable fibers such as alumina and silica. These can bepurchased from such sources as C & E Refractories, Buffalo, N.Y.. A keyto the desired porosity is the fiber length. The fibers deposited by thecurrent process should be less than 0.25 inch and preferably about 0.125inch in length (average fiber length). The fibers are typicallydeposited on the screen at a thickness of about 0.25 to about 0.5 inchthick. (If the coating is too thin the screen will heat up duringoperation). The mesh screen is typically formed into a cylinder with anend cap which is preferably circular, although square, rectangular, oroval cross sectional area containers are also usable. It should also benoted that, e.g. such things as donut or spherical shapes can be useddepending on the heat exchanger shape.

The ceramic fibers are typically about 2 inches long as purchased andtheir ultimate particle size is determined by chop-time. They aretypically chopped to the proper length in a conventional high speedchopper (e.g. Oster blender). The ceramic fibers are typically a mixtureof alumina and silica, with up to 70% by weight silica, and typically51% alumina and 49% silica.

The ceramic fibers are deposited out of a slurry which must be uniformlydispersed in order to produce a uniform coating with uniform emissiveproperties. Use of a sparger best produces such a mixture. By spargingthe container containing the ceramic fiber slurry with a non-reactivegas such as air pumped (typically at about 10 to 15 p.s.i. mercury)through a plate or tubing at the bottom of the vat, the mixture isbetter kept in suspension providing for uniform distribution of thecomponents in the mixture onto the screen. A system of paddles may alsobe used. Also, by virtue of the sparging in this fashion it is believedthat small bubbles may be trapped in the fiber matrix as it isdeposited, further contributing to the porosity of the mat on the burnerscreen. This assists in obtaining the low pressure drop. Otherdeposition methods can also be used, e.g. spraying, but uniform coatingis essential.

Polymeric particles which decompose upon heating are admixed with theceramic fibers in order to produce the requisite porosity after beingburned off. Acrylic polymers have been found to be best suited to this,with polymethylmethacrylate being preferred. These particles aretypically 20 to 40 mesh (0.18 to 0.2 inch in diameter). If the particlesare significantly smaller than this, it appears that the pressure dropwill increase, while if the particles are significantly larger thanthis, the structural integrity of the burner will suffer. The particlesassist the fibers in flowing during deposition almost acting as alubricant further providing for more uniform fiber laying.

The polymeric material is designed to ideally burn off at about 275° C.Complete burning can be detected as the film on the outside of theburner goes through a black char to white stage. This can be removed bybrushing off the outer surface, for example with a paint brush. Burningis typical accomplished by running a gas such as a natural gas-airmixture through the burner with an excess of air. The gas-air mixturesused in the burner for decomposing the polymer, operating the burner,and generating the pressure measurements recited throughout is typicallya mixture of more than 10 parts of air to 1 part of fuel (e.g. naturalgas - 96% methane, and a propane and butane mix). Typically 10% to 15%excess (required for stoichiometric combustion of the fuel) air is used,or approximately 11+ parts air per part of fuel.

Approximately 1/32 to 1/64 of an inch is all of the surface polymerwhich is completely removed. The remaining polymer contributes to thestructural integrity of the burner while still providing adequateporosity. The decomposition of the polymer takes place as a gradientthrough the thickness of the coating on the screen. As stated abovethere is no polymer on the surface of the article to a depth ofapproximately 1/32 to 1/64 of an inch. As you penetrate the coatingtowards the screen there is a greater concentration of polymer, althoughsufficient polymer is removed and decomposed throughout the thickness ofthe screen to provide the porosity as described herein.

The slurry is made up by acidifying an alumina suspension to keepeverything uniformly dispersed. Typically Versal™ alumina (KaiserChemicals, Baton Rouge, LA) is used at a pH of about 2. This produces ahomogeneous gel with about a 12% solids content. The fibers may beinitially soaked in an acid solution for suspension purposes.

Some aluminum nitrate may also be added to the dispersion to aid in gelformation and wetting of the fibers which can also contribute tostructural integrity.

It is important to chop the fibers in the alumina suspension. Choppingthe fibers in water and then adding them to the alumina does not producean adequate wetting of the fibers for good structural integrity of theburner. Typically concentrated Versal alumina is used and after choppingthe mix is diluted. This produces stronger bonding characteristicsbelieved to be the result of the alumina attaching to the fibers versusthe water attaching to the fibers and then the alumina having topenetrate the water layer. The soaking in Versal alumina is accomplishedlong enough to coat the fibers, typically 15 to 30 minutes.

The end cap can be produced by merely painting a ceramic slurry onto thetop of the screen thick enough (typically 1/8 to 1/4 inch) to eliminateany porosity allowing it to withstand the heat but not generate anyradiation.

The pressure drop produced is a critical part of the invention. It isimportant that the pressure drop be very low. In the past, fine fiberswere used to produce an even coating, but this resulted in a very highpressure drop across the surface. The problem of just switching tolonger fiber lengths is that this can produce adjacent bumps causinglocalized hot spots. Uniform mixing helps to avoid this. The pressuredrop of the burner according to the present invention is less than orequal to 0.4 inch (water) and preferably 0.1 inch to 0.4 inch. The lowpressure drop reduces power requirements for an inducer fan resulting inincreased energy efficiency. The other advantage of the burner accordingto the present invention is that the pressure drop cold (at startup)versus the pressure drop hot (at operating temperature) has a differenceof less than or equal to 0.2 inch and preferably less than or equal to0.15 inch (water column).

In FIG. 1 which is a perspective view partly broken away and partly incross-section of a cylindrical mesh screen (1) with a closed end (2),the coating (3) on the side wall portion (1) of the screen is a porousmaterial with a very low pressure drop. The end cap portion (4) isnonporous, i.e. permits no gas flow therethrough. A threaded coupling(5) is attached (e.g. by welding or bonding) at the open end (6) of thecylindrical screen. The coupling can be plastic, steel tube, sheetmetal, etc.

In FIG. 2 a flow chart of the method is described where in step "A"colloidal alumina is acidified typically to a pH of less than 2, e.g.using glacial acetic acid. (The carrier used throughout the process ispreferably water, although alcohols can be used.) This forms a gel atthis pH. In step "B", optionally, aluminum nitrate can be added to themixture as a solution in water to improve wetting and bonding of thefibers. In step "C" the fibers are also added to the mixture. Althoughthe fibers can be chopped in a carrier such as water and added to themixture it is preferable to add the fibers in the as-purchased oras-delivered length (typically 2 to 3 inches) and chop the fibers in theslurry. This seems to produce a better wetting of the fibers by themixture. At this point the pH is again adjusted, this time to neutralwith ammonium hydroxide to maintain the gel but reduce corrosiveness ofthe mixture.

In step "D" polymeric particles are added to the mixture and evenlydispersed therein. In step "E" the screen shown in FIG. 1 is immersed inthe solution and a constant, controlled vacuum pulled (typically -5 to-20 inches mercury) to deposit an even coating on the substrate(typically less than 15 seconds). This is followed in step "F" by dryingthe deposited coating. This drying takes place over the course ofapproximately an hour in an oven starting at room temperature and endingat a temperature of approximately 325° C. over the course of thisgradient the water or other carrier is first removed followed bysublimation of the particles, particularly on the inner surface of thecoating. Finally, in step "G" polymeric particles at the surface areburned off providing a porous coating. This burning takes place byoperating the burner at its standard operating-firing conditions. Thisis typically done under a nitrogen gas or other inert atmosphere.

EXAMPLE

In the following example the ingredients make approximately 3000 gramsof solution, which is sufficient to coat a burner screen 2 inches indiameter and 12 inches long (0.8 square foot surface area).

Procedure to make a double batch:

(1) Prepare acidified Versal 900 by mixing 120 g of powder to 880 gwater in blender and then acidify to pH<2 using glacial acetic acid. Agel should form at this pH.

(2) Prepare 14% Al(No₃)₃ by mixing 140g of the nitrate to 860 g of waterin a volumetric flask.

(3) Take 60 g of #1 above and add 200 g of #2 above and mix for oneminute on high speed in Oster blender.

(4) Add 600 g of water to blender and mix at high speed for one minute.Empty all solution into another container.

(5) Weigh 10 g of fiber and 200 to 250 g of mixture from number 4 aboveinto blender container and blend at high speed for approximately 40seconds. This is repeated 4 times (for a total of 40 g of fiber). Thetarget fiber length is approximately 1/8 inch. Typically after blendingif the slurry is lumpy you must blend for a longer period of time. Ifwhen applying the fibers to the screen they do not stick, it is anindication that a shorter blend time must be used.

(6) Dilute to 3000 ml with water

(7) Add 200 grams of methyl methacrylate beads and mix (paddles orsparger) until beads are evenly dispersed.

(8) Add indicator (methyl red or bromocresyl purple) and adjust pH toneutral (6.5) with ammonium hydroxide. Color change will be from red orpurple to yellow.

(9) Pour solution into vacuum forming vessel of adequate size and useair sparger or paddles to keep fibers/methacrylate evenly mixed insolution.

(10) Form burner while solution is being mixed or within minutes (e.g.10) after mixing.

(11) Dry burner at room temperature for 60 minutes.

(12) Sublime methyl methacrylate in oven at 325° C. temperature for atleast 8 hours.

(13) Ignite the burner at operating conditions allowing the methacrylateon the outer surface to burn off completely.

(14) Brush off the white film of alumina binder from the outer surfaceof the burner.

In addition to having a low pressure drop both during operation and coldversus hot, the infrared burners of the present invention havestructural integrity, and ability to generate radiant energy at a highrate. This makes for a very energy efficient system. Examples of theadvantages a low pressure drop cold versus hot produces are: (1) onlyone inductor fan is necessary for both startup and normal operatingconditions in a furnace-type environment for this type of a burner; (2)only one sensor is needed for the fans speed; (3) smaller inductor fancan be used; (4) a multi-speed motor is not necessary; and (5) there arefewer control complications in such a system. It should also be notedthat with a burner according to the present the structural integrity hasresulted in over 4,000 hours of testing without structural integrityfailure. One of the keys to the present invention is the balance ofproperties obtained, a balance of surface smoothness, structuralintegrity, and the pressure drops recited.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and scope of the claimed invention.

What is claimed is:
 1. An infrared burner comprising a corrosionresistant mesh screen having deposited thereon a thick porous coating ofceramic fibers having structural integrity, ability to generate radiantenergy at the rate of 80,000 BTU to 100,000 BTU per sq.ft. per hour, apressure drop during operation of up to 0.4 inch of water, and apressure drop cold versus hot of up to 0.2 inch of water.
 2. The burnerof claim 1 wherein the coating is 0.25 inch to 0.5 inch thick, thepressure drop during operation is 0.1 to 0.4 inch of water and thepressure drop cold versus hot is up to 0.15 inch of water.